Spong, in a copending application entitled, "Information Record and Related Recording and Playback Apparatus and Methods", U.S. Pat. No. 4,097,895, issued June 27, 1978 and incorporated herein by reference, describes an ablative recording system whereby a focussed modulated light beam, such as a laser beam, is directed at an ablative recording medium. The recording medium comprises a light reflecting material coated with a light absorbing material on a substrate. The thickness of the light absorbing layer is chosen to reduce the reflectivity to a minimum value so that a maximum of light energy impinging on it is retained therein and is converted to thermal energy. This thermal energy causes the light absorbing material in the area struck by the light to ablate, thereby exposing selected portions of the light reflecting layer. During readout, the contrast between the light reflected from the absorbing layer, which is at the reflection minimum, and the light reflecting layer is detected.
Ongoing work in this area has resulted in the improved performance of the materials employed. Thus, is an illustrative embodiment of this recording medium, a substrate which is a flat, smooth non-conductor of heat is coated with a thin layer of a light reflecting material such as aluminum. The aluminum layer is passivated as described in a copending application entitled, "Ablative Optical Recording Medium" by Bartolini et al, Ser. No. 668,504, filed Mar. 19, 1976. The passivated aluminum layer is in turn coated with a layer of an organic light absorbing material such as 4-phenylazo-1-naphthylamine as described in Bloom et al, "Ablative Optical Recording Medium", U.S. Pat. No. 4,023,185.
Alternatively, the light reflecting layer is coated with a transparent dielectric material such as silicon dioxide. A thin layer of a metal is coated thereon to serve as the light absorbing layer. This configuration is described by Bell in U.S. Pat. No. 4,216,501 issued Aug. 5, 1980 and entitled, "OPTICAL ANTI-REFLECTIVE INFORMATION RECORD". Titanium is the metal most frequently used for this application.
When an organic dye is used as the light absorbing layer, a continuing problem is the mechanical delicacy of the dye layer. A protective coating with good mechanical properties would make it easier to handle the recording medium without damage to the dye layer. In addition, an overcoat layer would protect the dye or metal light absorbing layer from reacting with the atmosphere, thereby extending the lifetime of the recording medium.
Another problem affecting both configurations of the recording medium is the effect of surface dust which precipitates onto the medium from the environment and causes signal defects or dropouts on readout or recording. As each dust particle moves under the focussed spot of the recording laser beam, it effectively shadows that portion of the track on which it lies, thus preventing the formation of information pits for the segment of the picture signal. On playback the section of recorded track which contained the dust particle exhibits a picture defect or dropout caused by the temporary loss of information. An improved recording medium would make it possible to remove dust safely without abrading or otherwise disturbing the light absorbing layer.